MR sensors are used as the magnetic read sensors or "heads" in magnetic recording data storage systems, such as magnetic recording disk drives. An MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage. Conventional MR sensors based on the AMR effect thus provide an essentially analog signal output, wherein the resistance and hence signal output is directly related to the strength of the magnetic field being sensed.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers. This GMR effect has also been observed for these types of multilayer structures, but wherein the ferromagnetic layers have a single crystalline structure and thus exhibit uniaxial magnetic anisotropy, as described in U.S. Pat. No. 5,134,533 and by K. Inomata, et al., J. Appl. Phys. 74 (6), 15 Sep. 1993. The physical origin of the GMR effect is that the application of an external magnetic field causes a reorientation of all of the magnetic moments of the ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus a change in the electrical resistance of the multilayered structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. MR sensors based on the GMR effect also provide an essentially analog signal output.
In a magnetic recording system data is stored in a thin data layer, typically a cobalt-based magnetic alloy, deposited on a substrate, such as a disk blank used in magnetic recording disks. Data is stored in the form of magnetic domains or "bits" in circumferential concentric tracks in the magnetic layer on the disk. The data is written by an inductive write head comprised of a magnetically soft magnetic pole, magnetized by passing a current through a thin film coil surrounding the pole. The written data is read by a conventional MR read head based on the AMR effect. The amount of data that can be stored in such a system is limited by the size of the magnetic bit, i.e., the smallest unit of recorded information, and the data area available on the disk.
Magnetic recording data storage technologies, particularly magnetic disk drive technologies, have undergone enormous increases in stored data per unit area of media (areal data density). This has occurred primarily by reducing the size of the magnetic bit through a reduction in the size of the read and write heads and a reduction in the head-disk spacing. At the same time, the size of the disk drive and thus the size of the disks has continued to decrease. This results in an ever decreasing amount of magnetic real estate available for the data. In conventional disk drives the dam is stored in a single data layer on each side of the disk substrate.
It would be advantageous to increase the areal data density not by reducing the size of a single magnetic bit, but by use of a plurality of independent, magnetically isolated magnetic data layers formed on the disk substrate. However it is not possible to read the data from multiple magnetic data layers simultaneously, using either conventional MR sensors based on the AMR effect or proposed MR sensors based on the GMR effect, because such sensors produce an analog output signal that is not capable of directly distinguishing between the discrete magnetic field levels that are generated by the superposed data from the multiple data layers.